Have you ever met an advanced skills teacher, or AST as they are more commonly known? You could be forgiven if you haven't, since they only make up about 1 per cent of the workforce.
To put it another way, even if they were all deployed in schools there would only be one for around every six schools overall. In the primary sector, the figure is little more than one AST for every 20 schools.
You are least likely to encounter an AST if your school is in the Yorkshire and Humber region, where there are only 5.5 ASTs per 1,000 teachers.
These subject specialists and paragons of teaching and learning who did not want to be sidetracked away from their specialism and into leadership posts are most common in the East Midlands and London, where there were 10 ASTs per 1,000 teachers this January.
In the secondary sector, ASTs are most likely to be found teaching maths, English or the sciences and least likely in humanities and special needs, an area where their expertise might be of considerable use.
Despite being re-launched several times by Labour ministers, the grade never really caught on, partly because the original concept required ASTs to carry out work beyond the school which appointed them, so that their expertise could be shared.
In an era of competition, the concept of lending your best teacher to another school did not find favour with everyone. As a result, numbers have barely changed over the past five years, increasing by only 300 or so from 3,780 in 2005 to 4,090 this year.
But even that is probably better than the "excellent teacher" grade that is rarer than the proverbial hen's tooth.
John Howson is director of Education Data Surveys, part of TSL Education.